The present invention relates to a graphic input device.
Means for converting characters, drawings and other patterns entered, or written, by hand has customarily been implemented by a graphic input device in which a position of a pen on an input or writing surface of a tablet is detected electromagnetically to generate a coodinate signal indicative of coordinates of the pen position. The tablet is provided with two orthogonal groups of conductive loops in the input surface, while the pen for writing information is provided with a winding therein. When information is written on the input surface of the tablet, the electromagnetic coupling point between the pen and the winding is detected to generate a coordinate signal indicative of the pen position in terms of coordinates.
Referring to FIG. 1, a prior art graphic input device includes a tablet 1 having an input surface on which two groups of conductive loops 10 and 11 are arranged orthogonally to each other. A pen 5 has a winding 6 fixed in place thereinside. A control circuit 4 delivers a timing signal and a carrier signal to scanning circuits 3 and 2. In response to the input signals, the scanning circuits 3 and 2 deliver to the loops 10 and 11, respectively, multi-phase burst signals which are multi-phase pulse signals modulated by the carrier. When the multi-phase burst signal is applied to the loop 10 (or 11), a magnetic field develops on the input surface of the tablet 1 and propagates at a predetermined rate along the horizontal or X coordinate axis (or vertical or Y coordinate axis). As information is entered, or written, by hand on the tablet surface, the winding 6 inside the pen 5 is interlinked with the propagating magnetic field with the result that a voltage complementary to a change in field intensity is induced across the opposite ends of the winding 6.
The voltage induced by the winding 6 is amplified by an amplifier 7 the output of which is in turn applied to a detector 8. The detector 8 comprises an envelope detector including a diode and serves to detect an envelope of the output signal of the amplifier 7. A coordinate detector circuit 9, supplied with the detected signal from the detector 8, detects a phase of the detected signal to generate a coordinate signal which indicates an X coordinate (or Y coordinate) of the position of the pen 5 on the tablet 1. As shown, the coordinate detector circuit 9 is made up of a filter 91, a phase detector 92 and a coordinate generator 93. The filter 91 separates from the detected signal a fundamental harmonic component which is a sinusoidal component having the same period as the previously mentioned multi-phase pulse signal. The phase detector 92 to which the fundamental harmonic component is applied serves to generate a pulse which rises when the phase angle of the fundamental harmonic component is brought to a predetermined value (e.g. a phase in which the amplitude crosses zero from the negative toward the positive), the pulse being routed to the coordinate generator 93. The coordinate generator 93 generates a digital signal indicative of an interval between the rise time of the pulse output from the phase detector 92 and that of a reference pulse output from the control circuit 4. The reference pulse rises at a timing which indicates the origin of the X coordinate or that of the Y coordinate. The digital signal from the coordinate generator 93 is fed out as a coordinates signal. Since the interval from the rise of the reference pulse to the rise of the pulse output from the phase detector 92 is proportional to the value of the X or Y coordinate of the pen position, the coordinate signal represents a digital value proportional to the X or Y coordinate of the pen position.
The problem encountered with such a prior art graphic input device is that error is introduced into a value indicated by the coordinate signal depending upon the inclination of the pen 5 during input operation, as will be described.
Referring to FIGS. 2A and 2B, the pen 5 is shown in two different positions relative to the input surface of the tablet 1. The pen 5 is shown in FIG. 2A with its axis perpendicular to the input surface and, in FIG. 2B, with the axis inclined an angle .theta. relative to a line vertical to the input surface. The winding 6 in the pen 5 is positioned at a predetermined distance, or height, h from the pen tip and has an interlinkage surface with the magnetic field which is perpendicular to the axis of the pen 5. Where informtion is written with the pen 5 held upright as in FIG. 2A, the winding 6 is interlinked solely with the vertical component of the propagating field at a position which shares the same coordinate as the pen tip and at the height h above the input surface. Meanwhile, where the pen 5 is inclined as shown in FIG. 2B, the winding 6 is interlinked with both the vertical and horizontal components of the propagating magnetic field at a coordinate which is deviated by a complementary amount to the inclination.
The inclination of the pen 5 shown in FIGS. 2A and 2B effects detection of coordinates as represented by waveforms in FIG. 3. The dotted waveforms in FIG. 3 represent signals provided by the pen 5 in the upright position shown in FIG. 2A, while the solid waveforms represent signals provided by the pen 5 in the inclined position shown in FIG. 2B. Here, the tip of the pen 5 is located at the same coordinates. The detected signal is the signal output from the detector 8 shown in FIG. 1. The zero-phase component and the orthogonal component are set up by decomposing the detected signal into two orthogonal components. As shown, the waveform of the zero-phase component is symmetrical with respect to a positive peak of the detected signal, while that of the orthogonal component is inverse with respect to the same. Stated another way, assuming that a positive peak of the detected signal indicates a time at which the phase angle is zero, the zero phase component is a cosine series component of the detected signal and the orthogonal component, a sine series component. The detected signal is the sum of the zero phase component and the orthogonal component. The fundamental harmonic component is that of the detected signal and, therefore, equal to the sum of the fundamental harmonic components of the zero phase and orthogonal components.
A detected signal Do obtained with the pen 5 held in the upright position has a waveform which becomes symmetrical with respect to a peak of the amplitude at every scanning period. When decomposed into two components, the detected signal Do is made up of a zero phase component Po which is equal to the signal Do and an orthogonal component which is zero. Therefore, the fundamental harmonic component Fo has a cosine waveform which reaches a positive peak when the amplitude of the signal Do is at a peak.
On the other hand, where the pen 5 is inclined, a detected signal D.theta. appears with a lag relative to the signal Do and loses symmetry, due to a lag originating from the previously described deviation in coordinate between the winding 6 and the pen tip and the interlinkage of the winding 6 with the horizontal component of the propagating magnetic field. A zero phase component P.theta., provided by decomposing the signal D.theta., reaches a peak at a time t.sub.n (or t.sub.n+1) which is deviated from the peak of the zero phase component Po. Also, an orthogonal component Q.theta. appears due to the asymmetry of the waveform of the detected signal D.theta.. The waveform of the orthogonal component Q.theta. is an alternating waveform which inverts from the negative to the positive just before the time t.sub.n (or t.sub.n+1) and, therefore, the primary Fourier coefficient of its sine series is very small and almost zero. It follows that the fundamental harmonic component F.theta. has a cosine waveform substantially common in phase with the fundamental harmonic component of the zero phase component P.theta., i.e. cosine waveform having a peak at the time t.sub.n (or t.sub.n+1).
The filter 91 shown in FIG. 1 separates the fundamental harmonic component of FIG. 3 and delivers it to the phase detector 92. Upon receipt of the fundamental harmonic component Fo, the phase detector 92 generates a pulse which rises at a time .tau..sub.0 and, upon receipt of the fundamental harmonic component F.theta., a pulse which rises at a time .tau..theta.. The time .tau..theta. is deviated from the time .tau.o by an amount substantially complementary to the time lag which orginates from the inclination of the pen 5. Therefore, although the tip of the pen 5 may remain in the same position, any change in the inclination of the pen 5 introduces error into a value indicated by the coordinate signal.
The relationship between an angle of inclination .theta. of the pen 5 and a coordinate deviation d is shown in FIG. 4. The coordinate deviation d implies a deviation of a value indicated by the coordinate signal which resulted from a change in the angle .theta. for the same position of the pen tip on the input surface. A solid line 12 pertains to a case wherein the winding 6 is at a small height h and mounted in the pen 5 adjacent to the tip, while a dotted line 13 pertains to a case wherein the winding 6 is located higher than the winding associated with the solid line 12. As shown in FIG. 4, the primary requisite for the coordinate deviation d to be reduced is that the winding 6 be located as near to the pen tip as possible.
FIG. 5 is a side elevation showing an exemplary structure of the pen 5. Since the winding 6 is located close to the tip 51 of the pen 5 for the reason described above, the outside diameter of the pen 5 cannot be decreased in its portion which accommodates the winding 6. This does not allow the operator to see the pen tip 51 and, thereby, makes it difficult to enter fine and/or complicated patterns.